Mouse hepatitis virus (MHV) infection of the mouse CNS provides an animal model for the study of chronic demyelinating disease such as multiple sclerosis (MS). The MHV-A59 and MHV-4 (JHM) strains cause acute encephalitis followed by chronic demyelinating disease in C57Bl/6 mice. Our long-term goal is to understand the mechanism of MHV pathogenesis. In this proposal, we will focus on the role of the virus-specific CD8+ T cell response in CNS pathogenesis. It is clear that the CD8+ T cell response plays a crucial role in viral clearance from the CNS during the acute infection. Studies of various isolates and mutants of MHV suggest that some level of encephalitis is necessary for the later development of demyelination, which occurs in the absence of detectable infectious virus but in the presence of viral RNA. Our hypothesis is that the CD8+T cell response is important in achieving a balance between virus replication/spread versus clearance by the host immune response and that this balance (which determines the level of encephalitis) is necessary to set up the conditions that lead to viral RNA persistence and demyelination. We will investigate how alteration of the CD8+ response effects acute infection and progression into chronic demyelination. We will use isogenic viruses that differ only in the spike gene, expressing either the MHV-4 spike or the A59 spike within the A59 background. While both viruses induce demyelination, they differ in the extent of encephalitis induced and in CD8+ T cell epitopes; thus we suggest that each has evolved a balance of viral replication/spread versus clearance in a different way, both resulting in demyelination. While the MHV-4 spike contains the immunodominant S510-518 (H-2Db) epitope as well as the subdominant S598-605 (H-2Kb), the A59 spike expresses only S598-605. We have selected recombinant viruses with alterations in S510-518 and will select viruses with inactivating mutations in both epitopes. We will use these viruses to test our prediction that alteration of one of both epitopes will alter the pathogenesis of acute and chronic disease. Finally, using a complementary approach, we will develop a model in which CD8 deficient mice will be infected with a recombinant MHV expressing a foreign CD8+T cell epitope. We will determine the effects of transferring various numbers of epitopes specific CD8+ T cells on clearance and pathogenesis.